Oxide transparent conductive films have low resistance and high transmittance in the visible light region, and are used as display devices such as liquid crystals, and light receiving elements such as solar cells. Among oxide transparent conductive films, indium oxide films are widely used as tin-added ITO films or zinc-added IZO films.
When utilizing an oxide transparent conductive film for a solar cell, where it is important to have optical transparency across a wide wavelength range, it is desirable to further improve the optical transparency not only in the visible light region but also in the infrared region. While measures have been taken to reduce tin content of ITO films in order to satisfy this need, this strategy has tended to result in lower reliability.
ITiO films that have titanium added instead of tin can improve optical transparency in the infrared region, but their reliability is inadequate. It has therefore been desirable to obtain both higher optical transparency in the infrared region and higher reliability.
PTL 1 discloses a high-resistance indium oxide-based transparent conductive film with tin and an insulator added, having a resistivity of about 0.8 to 10×10−3 Ωcm. Although strontium is mentioned as an example of the insulator, no disclosure thereof is provided in the examples, and absolutely nothing is mentioned regarding the composition and characteristics.
Also, PTL 2 discloses an indium oxide-based transparent conductive film with addition of tin and strontium. It is stated therein that an amorphous film can be easily obtained and that the amorphous film can be easily patterned by weak acid etching, and also that the readily crystallized film has low resistance and high transmittance. However, nothing is mentioned regarding optical transparency in the infrared region or reliability, which are important for use in solar cells where optical transparency is important. With sputtering targets used for such film formation, minimizing generation of anomalous discharge during the sputtering helps minimize the reduction in yield caused by particle fly-off, and contributes to productivity. While this is considered to be a very important issue, no disclosure is provided regarding anomalous discharge during sputtering.
Specifically, with a conventional sputtering target to be used for film formation, it has been difficult to adequately reduce generation of anomalous discharge during sputtering. This results in the problem of reduction in yield due to particle fly-off during anomalous discharge, and lower productivity. It has therefore been a goal to establish a technique for further suppressing anomalous discharge.
For thin-film solar cells, the most important issue is to achieve high photoelectric conversion efficiency and high heat-proof humidity resistance. Consequently, the transparent electrodes used therein must also contribute to increased photoelectric conversion efficiency and heat-proof humidity resistance of solar cells.
As existing transparent electrodes for thin-film solar cells there are used oxide transparent conductive films such as fluorine-added tin oxide films, tin- or zinc-added indium oxide films and aluminum-, gallium- and/or boron-added zinc oxide films. Tin-added indium oxide films, in particular, have low resistance and excellent durability as disclosed in PTL 3 and PTL 4. They have therefore been suitably used as transparent electrodes in thin-film solar cells.
However, tin-added indium oxide films have high carrier-caused reflection in the near-infrared region, and hence low transmittance. For this reason, they cannot efficiently utilize sunlight across a wide wavelength range, and their low photoelectric conversion efficiency has been a problem. Films that can increase photoelectric conversion efficiency have therefore been desired.